Process for making tacky acrylate microparticles and use therefor

ABSTRACT

The present invention pertains to a process for preparing clusters of infusible, inherently tacky, elastomeric, solvent-insoluble, solvent-dispersible polymeric microparticles by aqueous emulsion polymerization. The invention also pertains to an adhesive comprising the polymeric microparticles and to a repeatedly reusable pressure sensitive sheet material having the elastomeric microparticles coated thereon.

BACKGROUND OF THE INVENTION

In the mid-1970's, the Minnesota Manufacturing and Mining Corp. (3M)introduced a product called POST-IT™ which is made by coating pressuresensitive microsphere adhesives on a sheet material, such as paper,which can be repeatedly adhered to and removed from a substrate withoutdamaging the surface of the substrate. The microspheres are prepared bysuspension polymerization and employ oil soluble initiators (U.S. Pat.No. 3,691,140 issued to Silver and U.S. Pat. No. 4,166,152 issued toBaker). Emulsion polymerization employing water-soluble initiators,according to Silver, can result in a substantial amount of latex havingextremely small particle size which are difficult to recover and,further, have undesirable solvent solubility for many applications asadhesives. Other processes for making microspheres by suspensionpolymerization techniques using oil-soluble initiators are described inU.S. Pat. Nos. 4,495,318 and 4,598,112 issued to Howard, U.S. Pat. No.4,810,783 issued to Mallya, and U.S. Pat. No. 4,839,416 issued toOrenstein.

In most cases, the adhesive microspheres must first be recovered fromliquid medium before they can be re-dispersed in proper organic solventsand coated onto an appropriate surface. The suspension polymerizationprocess described in the above patents produces a finely dispersedsuspension of adhesive microspheres which are difficult and laborintensive to recover from the liquid medium. Recovery techniquestypically require a compatible solvent, such as methanol, to coagulatebefore filtration, or mechanical force, such as centrifuge, to removethe microspheres from suspension.

Recently, Young (U.S. Pat. No. 4,833,179) reported a method of producingpolymeric adhesive particles by suspension polymerization which could beeasily collected by gravity filtration. The adhesive particles haveadditional modifying moieties, such as macromolecular monomers,hydrophobic silicas, or reactive zinc salts, which distort theproperties of the adhesive.

It would therefore be desirable to develop a process for producingelastomeric microparticles that can be easily recoverable from a polymersuspension, in a form suitable for use as an adhesive.

SUMMARY OF THE INVENTION

The present invention pertains to a process for preparing clusters ofinfusible, inherently tacky, elastomeric, solvent-insoluble,solvent-dispersible polymeric microparticles by aqueous emulsionpolymerization. The invention also pertains to an adhesive comprisingthe polymeric microparticles and to a repeatedly reusable pressuresensitive sheet material having the elastomeric microparticles coatedthereon.

According to the invention, an aqueous suspension comprising monomershaving at least one substantially water-insoluble ester of alkylacrylate or methacrylate, at least one stabilizer and at least oneemulsifier is polymerized with a water-soluble, oil-insoluble redoxpolymerization initiator, whereby clusters of elastomeric microparticlesare formed. The clusters can be recovered from suspension by filtrationand the microparticles resuspended in an appropriate solvent. Thedispersed microparticles can then be coated onto paper or other sheetmaterial surfaces as an adhesive for self-adherent, removable andrepositionable use.

DETAILED DESCRIPTION OF THE INVENTION

According to this invention, clusters of inherently tacky, elastomeric,solvent-insoluble, solvent-dispersible polymeric microparticles areprepared by aqueous emulsion polymerization. The polymerization isinitiated by reacting an aqueous suspension comprising monomers havingat least one substantially water-insoluble ester of alkyl acrylate ormethacrylate, a stabilizer and an emulsifier with a water-soluble redoxpolymerization initiator to produce clusters of elastomericmicroparticles. During the polymerization, the monomers formmicroparticles which gradually coagulate to form clusters. The clustershave a macroporous structure and thus, they can be readily recoveredfrom suspension by filtration, decantation, or other means by which theclusters can be removed from the aqueous phase. Preferably, thepolymerized microparticles form clusters having an average size aboveabout 300 microns, preferably from about 300 to about 2,000 microns. Themicroparticles are spherical and have diameters in the range of fromabout 5 to about 200 microns.

After the clusters of elastomeric microparticles are formed andrecovered from suspension, the clusters can be resuspended in a suitableorganic solvent, such as n-hexane. The microparticles can then bedissociated from the clusters by mechanical means such as by agitation.Once the microparticles are dispersed within the organic solvent, theycan be coated onto an appropriate surface, such as paper or other sheetmaterial for use as an inherently tacky, elastomeric, infusible pressuresensitive adhesive. The adhesives are self-adherent and can berepeatedly removed and/or repositioned on a surface withoutsubstantially altering or destroying the adherent properties of theadhesive. Further, the pressure sensitive adhesives of this inventionwill not damage the surface to which it is attached, upon removal of theadhesive. The pressure sensitive adhesive produced according to thisinvention can be repeatedly used as removable labels, tapes, removablebulletin boards, etc. Other uses in which it would be desirable torepeatedly adhere and remove an item from a surface without damaging thesurface are included within the scope of this invention.

Monomers which can be homopolymerized or copolymerized according to thisinvention comprise at least one water-insoluble ester of alkyl acrylateor methacrylate, namely isodecyl acrylate, isooctyl acrylate, 2-ethylhexyl acrylate, n-butyl acrylate and isodecyl methacrylate. Othermonomers which can be used for copolymerization are unsaturated mono-and dicarboxylic acids such as methacrylic acid, acrylic acid, fumaricacid, and the like. Other comonomers such as acrylonitrile, dibutylfumarate, vinyl acetate, and the like also can be used. The amount ofacrylate or methacrylate monomer used is from about 95 percent to about100 percent, and correspondingly the amount of other monomers used is upto about 5 percent. In one embodiment, a preferred adhesive willcomprise 99% by weight isodecyl acrylate (IDA) and 1% by weightmethacrylic acid. This adhesive is prepared by aqueous emulsionpolymerization described herein and in detail in the examples.

To enhance the cohesive strength of the microparticles in order toachieve infusibility, the monomers are internally cross-linked to someextent using a small amount of a multifunctional cross-linker, such asmulti-functional acrylate, triallyl cyanurate, divinyl benzene and thelike. The amount of crosslinker used can be from about 0.5 to about 1.5percent by weight of the monomers, preferably about 1.0 percent byweight of the monomers.

Polymerization occurs by using a water-soluble redox system initiatorwhich comprises a pair of oxidizing and reducing agents. The oxidizingagent is a persulfate such as ammonium persulfate, sodium persulfate,and the like, preferably ammonium persulfate. The concentration of thepersulfate is from about 0.25% to about 1.0% by weight of the monomers,and preferably about 0.75% by weight of the monomers. The reducing agentis ethylenedinitrilotetraacetic acid sodium iron(+3) salt (EDTA-Fe(3+))which needs to be reduced by a second reducing agent, such as sodiumformaldehyde sulfoxylate. The concentration of EDTA-Fe(3+) and sodiumformaldehyde sulfoxylate is about 0.05 percent and about 0.5 percent byweight of the monomers, respectively.

A stabilizer should also be employed to prevent droplets of the monomersfrom coalescing in suspension. The stabilizer includes but is notlimited to methyl cellulose, hydroxy propyl cellulose, hydroxy propylmethyl cellulose and ethyl cellulose; however, other known stabilizerscan be used. The amount of stabilizer used in the polymerizationreaction is up to about 2.0 percent by weight of the monomers,preferably about 0.4 percent to about 1.0 percent by weight of themonomers.

In addition to the stabilizer, at least one emulsifier should beincluded in the suspension. The emulsifier can be anionic, such assodium dodecyl benzene sulfonate, sodium salts of alkyl aryl ethersulfonates and the like, or non-ionic such as alkyl aryl polyetheralcohols. Both anionic and non-ionic emulsifiers can be employedsimultaneously in the reaction. The amount of emulsifier (either anionicor non-ionic) used in the polymerization typically ranges from about 0.2to about 2.0 percent by weight of the monomers. Preferably, the amountof emulsifier is from about 0.4 percent to about 1.5 percent by weightof the monomers.

Other amounts of stabilizer or emulsifier can be used in this invention.However, use of high concentrations of stabilizer or emulsifier shouldbe avoided because such concentrations may result in finely dispersedadhesive microparticles that are difficult to recover from thesuspension by filtration.

This invention is further illustrated by the following non-limitingexamples:

EXAMPLE 1

A 2 liter cylindrical glass reactor equipped with thermometer, refluxcondenser, mechanical stirrer, and nitrogen inlet tube, was charged with17.1 grams of 28% by weight active sodium alkyl aryl polyethersulphonate (Rohm & Haas Company, Triton™ X-200), 700 grams of de-ionizedwater, 3 grams of ammonium persulfate, 0.2 grams of EDTA-Fe(3+), and 4grams sodium bicarbonate. The suspension was stirred at a speed of about500 rpm. A nitrogen purge was kept over the batch throughout thereaction period. To the solution were added a premix of 392 grams ofisodecyl acrylate, 4 grams of methacrylic acid and 4 grams of divinylbenzene and agitated for about 10 minutes. 80 grams of 2% by weighthydroxy propyl methyl cellulose (Aldrich Chemical Company, Inc. CatalogNo. 20,032-8) were then added to the suspension and agitated for about 5minutes. 2 grams of sodium formaldehyde sulfoxylate and 100 grams ofde-ionized water were then added into the reactor and the whole mixturewas allowed to polymerize for 90 minutes. During this period, thetemperature of the mixture raised spontaneously to about 37° C. due toexothermic reaction, and microparticle clusters appeared and grew insize. The reaction temperature was then raised to 72° C. for two hoursand then allowed to cool to room temperature after the reaction wascompleted.

The final mixture was recovered from the suspension by filtering througha piece of cheese cloth. The resultant clusters were about 67% by weightin solid, and the size of the clusters were from about 1,000 to about2,000 microns. The wet clusters were dried in an oven. The yield ofdried clusters was 88% by weight.

20 grams of resultant microparticle clusters were placed into a flaskwith 80 grams of n-hexane. The clusters were agitated in the solventuntil the microparticles were separated from the clusters and dispersedwithin the solvent. The microparticle dispersion was then deposited ontopaper and dried.

The coated paper was inspected with Scanning Electron Microscopy (SEM)for size and particle distribution. The majority of particles haddiameters ranging from about 5 to about 200 microns. The coated paperwas adhered to and repeatedly removed from paper substrates and did notshow any visible damage on their surfaces.

EXAMPLE 2

The procedure of Example 1 was repeated except that isodecyl acrylatewas replaced with isooctyl acrylate. The resulted clusters have sizefrom about 1000 microns to about 2000 microns. The yield is 85% byweight.

EXAMPLE 3

The procedure of Example 1 was repeated except that isodecyl acrylatewas replaced with 2-ethyl hexyl acrylate. The resulted clusters havesize from about 1000 microns to about 2000 microns The yield is 80% byweight.

Equivalents

Those skilled in the art will recognize or be able to ascertain, usingno more than routine experimentation, many equivalents to the specificembodiments of the invention described herein. Such equivalents areintended to be encompassed by the following claims:

We claim:
 1. A process for preparing clusters of infusible, solvent-insoluble, solvent-dispersible, inherently tacky, elastomeric microparticles, comprising reacting a water-soluble redox initiator with an aqueous suspension comprising at least one stabilizer, at least one crosslinking agent, at least one emulsifier in an amount sufficient to produce monomer droplets having a particle size capable, upon polymerization, of forming microparticles individually being of from about 5 to about 200 microns in diameter, and monomers having at least one substantially water-insoluble ester of alkyl acrylate or methacrylate, under agitating conditions to produce a dispersion of the monomers in suspension, whereby the dispersed monomers polymerize forming clusters of elastomeric microparticles.
 2. The process of claim 1, wherein the ester is isodecyl acrylate, isooctyl acrylate, 2-ethyl hexyl acrylate, n-butyl acrylate or isodecyl methacrylate.
 3. The process of claim 1, wherein the ester is present at from about 95 percent to about 100 percent by weight of the monomers.
 4. The process of claim 1, wherein the oxidizing agent of the redox initiator is ammonium persulfate or sodium persulfate.
 5. The process of claim 4, wherein the ammonium persulfate or sodium persulfate is present at from about 0.25 to about 1 percent by weight of the monomers.
 6. The process of claim 4, wherein the redox initiator further comprises ethylenedinitrilotetraacetic acid sodium iron(+3) salt (EDTA-Fe(3+)) and a second reducing agent.
 7. The process of claim 6, wherein the EDTA-Fe(3+) is present at about 0.05 percent by weight of the monomers.
 8. The process of claim 6, wherein the second reducing agent is sodium formaldehyde sulfoxylate.
 9. The process of claim 8, wherein sodium formaldehyde sulfoxylate is present at about 0.5 percent by weight of the monomers.
 10. The process of claim 1, wherein the stabilizer is methyl cellulose, hydroxy propyl cellulose, hydroxy propyl methyl cellulose or ethyl cellulose.
 11. The process of claim 10, wherein the stabilizer is present at up to about 2 percent by weight of the monomers.
 12. The process of claim 1, wherein the emulsifier is sodium dodecyl benzene sulfonate, sodium salts of alkyl aryl ether sulfonate, alkyl aryl ether sulfonate or alkyl aryl polyether alcohol.
 13. The process of claim 12, wherein the emulsifier is present from about 0.5 to about 1.5 percent by weight of the monomers.
 14. The process of claim 1, further comprising removing the elastomeric microparticle clusters from suspension by filtration.
 15. The process of claim 1, wherein the elastomeric microparticles comprise from about 95 percent to about 99 percent by weight isodecyl acrylate and from about 1 percent to about 5 percent methacrylic acid. 